Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an air-fuel ratio control apparatus that controls an air-fuel ratio corresponding to each cylinder of an internal combustion engine having a plurality of cylinders, using a controlled object model obtained by modeling the internal combustion engine.
In an internal combustion engine having a plurality of cylinders, the purifying efficiency of a catalyst provided in an exhaust system of the engine decreases with an increase in air-fuel ratio differences between the plurality of cylinders, causing a deterioration in exhaust characteristics. Therefore, a technique for estimating an air-fuel ratio corresponding to each cylinder and correcting the air-fuel ratio cylinder by cylinder is described in Japanese Patent Laid-open No. Hei 10-54279, for example. According to this publication, a method is shown for estimating an air-fuel ratio corresponding to each cylinder according to an output from a single air-fuel ratio sensor provided at a position downstream of a joining portion of the exhaust manifold connected to cylinders of the engine. More specifically, the air-fuel ratio corresponding to each cylinder is estimated by an observer for observing an internal condition of an exhaust system of the engine according to a model representing the behavior of the exhaust system.
Recently, a self-tuning regulator as described in Japanese Patent Laid-open No. Hei 11-73206, for example, is often used for the air-fuel ratio control of an internal combustion engine. The amount of computation for realizing the self-tuning regulator by a microcomputer is greater compared with a PID (Proportional Integral Derivative) control. Therefore, if the computation for realizing the observer is further necessary, the amount of computation further increases.
Accordingly, it has been desired to provide a method of more simply determining whether the air-fuel ratio differences between a plurality of cylinders has increased.